I spotted this today.... a combination of rewite and proxy to use another machine stuck on a local port as a backend server. Hope this adds more light.
Code:
<virtual host 192.168.0.3>
servername www.frontend.com
RewriteEngine On
RewriteOptions 'inherit'
RewriteRule ^/(.*)$ http://192.168.0.4:8001/$1 [proxy]
</virtual host>